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Model Diagnostics Discussion
(Caution: Version displayed is not the latest version. - Issued 1702Z Mar 18, 2018)
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Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
102 PM EDT Sun Mar 18 2018

Valid Mar 18/1200 UTC thru Mar 22/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air

12Z Model Evaluation...Including Model Preferences and Confidence

...Potent Shortwave Trough Rapidly Traversing CONUS from the Four
Corners the Mid Atlantic Coast Tuesday...
...Surface Low Developing in the Southern Plains...Moving into
Tennessee Valley Mon Ngt...Transfer to Coastal Low Tuesday...
Preference: Blend of 12Z GFS, 00Z UKMET/ECMWF
            Greatest weight on the ECMWF
Confidence: Average

Models still show important differences with this system beyond
12Z Monday, with some run-to-run inconsistencies in addition to
the model-to-model differences. The 12Z NAM continues to have the
strongest depiction of the trough, holding on to a closed low at
500mb well after the other models open it into a wave as the
trough axis gets east of the Mississippi River. This leads to an
overall slower progression on the NAM with the associated
vorticity maximum aloft situated to the northwest of other models.
Its surface low position is not far off the other models, but it
takes longer to complete the transfer to a coastal low. The lower
heights aloft lingering over the Mid Atlantic also lead to a
different trough structure as the secondary wave (discussed below)
arrives. For these reasons, the NAM is not preferred at this time.

Otherwise, the operational global models show reasonable
similarity. In fact, most of the isoheights aloft are contained
within the envelope of all the ensemble members from the GEFS,
ECMWF, and CMC, which suggests that any of them could be offering
a plausible scenario. One of the primary differences continues to
be with the 00Z CMC which shows slightly higher heights aloft
through much of the eastern CONUS in advance of the trough, and
ensemble sensitivity analysis shows that the surface low intensity
and track is sensitive to the strength of this ridging. Given that
the other models (including the NAM) are well clustered with less
ridging than is shown by the CMC, it is also not preferred. The
00Z UKMET, 12Z GFS, and 00Z ECMWF all seem plausible, so the
preference is a blend of the three, with a weight toward the ECMWF
as it is closest to the better ensemble clustering and the
ensemble means.

...Low Amplitude Shortwave Reaching California Today...then
Amplifying over Southern States Monday-Tuesday...
...Secondary Surface Cyclogenesis Along Coastal Carolinas by
Tuesday Night and Possibly up the East Coast...
Preference: Blend of 12Z GFS, 00Z ECMWF
Confidence: Slightly Below Average

As discussed with the first system (initial wave arriving on the
East Coast), model differences in the first 24-36 hours do play
important roles in how the models handle this system and the
secondary cyclogenesis along the East Coast. Given that the NAM
and CMC are not preferred for the initial system, they are also
not preferred here. The 00Z CMC in particular seems to carry
forward its bias for higher heights in showing more amplified
ridging ahead of this secondary wave. Ensemble sensitivity
analysis continues to show that a sharper downstream ridge
contributes to a deeper surface low closer to the East Coast.
Again, the CMC is within the envelope of ensemble spread, so it is
not an impossible scenario, but the model preference at this time
is weighted toward other global models.

Examining ensemble member distribution of surface low positions,
there are a considerable number of GEFS and ECMWF ensemble members
that are at least slightly to the left of the ensemble mean
positions (although not many as far left as the 00Z CMC). The 12Z
GFS and 00Z ECMWF continued a noticeable trend of pulling the
surface low track further northwest, and this is not uncommon
about 2-4 days prior to the verifying event. The GFS and ECMWF are
also very close to the current ensemble mean positions. Current
trends and ensemble distributions therefore suggest these models
have some room to nudge closer to the coast in subsequent runs.
The 00Z UKMET, meanwhile, has a flatter wave overall, with a
weaker surface low that is further offshore. Given that this is in
contrast to the trend and the ensemble information, the preference
is to lean toward a blend of the GFS and ECMWF at this time.

...Large Closed Low over the Northwest Rapidly Breaking Down Today
and Tonight into Small Distinct Waves...
...Limited Motion of the Waves in Collapsing Shear Axis Mon-Tue...
...Remnant Vort Maxes Feeding into East Coast Trough Wednesday...
Preference: General Model Blend
            Less weight on 12Z GFS; More on 00Z ECMWF
Confidence: Slightly Below Average

The chaotic breakdown of the closed low and the associated
individual waves that result does not lend itself to a specific
model preference. There is some variety among the different
models, and even CMC, GEFS, and ECMWF ensemble means show a fair
amount of spread. Therefore, a general model blend is the
preference at this time. The one systematic bias relative to other
models appears to be the 12Z GFS, which advances the distinct
waves faster to the southeast across the Northwest and Northern
Plains. Therefore, slightly less weight is placed on the GFS in
this region, with a greater weight on the 00Z ECMWF.

...Closed Pacific Low near 35N/145W Drifting East and Eventually
Opening into a Wave near 135W by Wednesday Morning...
...Increasing Deep SW Flow and Atmospheric River into Srn CA...
Preference: General Model Blend; Greater Weight on 00Z ECMWF
Confidence: Slightly Above Average

The CMC appears to be showing its recent bias in amplifying the
downstream ridge much more than other models. Isoheights at 500mb
are as much as 2-3 degrees of latitude north of the GEFS and ECMWF
ensemble means along the West Coast by Tuesday morning. These
substantial differences would almost certainly affect the
placement of precipitation and other sensible weather impacts, and
thus the CMC is excluded from the preference for this system.

Otherwise, the remaining models are similar enough that a general
blend is preferred. The NAM and GFS do break down the closed low a
bit faster than the ECMWF, and a slower transition to an open wave
seems a little more reasonable given that the low is effectively
cut off from a lot of other influences through Tuesday morning. So
greater weight will be placed on the 00Z ECMWF.

...Shortwave Rounding Pacific Ridge over near Alaska by Tuesday...
...Digging Sharply South and Developing a Closed Low over the Gulf
of Alaska by Wednesday...
...Associated Pressure Falls and Possible Surface Low Development
NW of Vancouver Island...
Preference: General Model Blend; Greater Weight on 00Z ECMWF
Confidence: Slightly Below Average

The NAM and the GFS may break down the closed low further south
faster because they also show a faster southward progression of
the height falls associated with this developing low over the Gulf
of Alaska. This would exert more influence over the system to the
south earlier in the forecast period. Given the strength of the
closed low developing over the Gulf of Alaska -- agreed upon by
all models -- the expectation is that a slower progression would
also be favored here, which also argues for a lean toward the 00Z
ECMWF. Given uncertainties with the initial wave (over the NW
Pacific at the moment), the preference is for a general model
blend at the moment as individual models can't be entirely
discounted. However, a greater weight will be placed on the ECMWF
for reasons stated above.

Model trends at
500 mb forecasts at